Non-isotopically labelled oligonucleotides are essential components in many important molecular biology applications, such as PCR amplification, DNA sequencing, antisense transcriptional and translational control of gene expression, genetic analysis, and DNA probe-based diagnostic testing (Keller, 1993; Kricka, 1992). Fluorescence detection of fluorescent dye-labelled oligonucleotides is the basis for nucleic acid sequence detection assays such as Taqman.TM. (Livak, 1996), Molecular Beacons (Tyagi, 1996), genetic linkage mapping (Dib, 1996), and oligonucleotide-ligation assay (Grossman, 1994).
Two general methods for labeling synthetic oligonucleotides have been established. In a first method, referred to herein as the "two-step solution labelling method", a nucleophilic functionality, e.g. a primary aliphatic amine, is introduced at a labelling attachment site on an oligonucleotide, e.g. a 5' terminus. After automated, solid-support synthesis is complete, the oligonucleotide is cleaved from the support and all protecting groups are removed. The nucleophile-oligonucleotide is reacted with an excess of a label reagent containing an electrophilic moiety, e.g. isothiocyanate or activated ester, e.g. N-hydroxysuccinimide (NHS), under homogeneous solution conditions (Hermanson, 1996; Andrus, 1995).
In a second alternative method, referred to herein as the "direct labeling method", a label is directly incorporated into the oligonucleotide during or prior to synthesis (Mullah, 1998; Nelson, 1992). The direct labelling method is preferred because it (i) does not require a post-synthesis reaction step, thereby simplifying the synthesis of labelled polynucleotides; and (ii) avoids the problems associated with the low reaction yield (&lt;60%) typically encountered with the two-step solution labelling method, namely: (a) purification of the labeled oligonucleotide from excess label; (b) purification of the labeled oligonucleotide from unlabeled oligonucleotide; (c) high costs due to the low product yield and laborious analytical and purification procedures, and; (d) irreversible capping of the nucleophilic functionality during synthesis.
Certain fluorescent dyes and other labels have been finctionalized as phosphoramidite reagents for 5' labelling (Theisen, 1992). However, some labels, e.g., digoxigenin, rhodamine dyes, and cyanine dyes, are too unstable to survive the harsh conditions and reagents used in reagent preparation and oligonucleotide synthesis, cleavage and deprotection. Thus, whenever such labels are used in current solid phase synthesis protocols, they must be attached using the less preferred two-step solution labelling method.
Therefore it is desirable to provide methods and reagents to label oligonucleotides and analogs directly on a solid-support upon which they are synthesized, under conditions which are rapid, economical, and compatible with chemically-labile functionality.